Digital cameras and other imaging devices typically have a light sensing apparatus for capturing and storing images. For example, in one popular design, an array of photodiodes, typically arranged in a charge-coupled device (CCD) or on a complimentary metal-oxide semiconductor (CMOS) microchip, are used for capturing and storing images. Each photodiode and its associated circuitry, (the combination of which is often called an Active Pixel Sensor (APS) or more simply, a pixel), converts the light intensity detected at the photodiode into a voltage signal that can be digitized for storage, reproduction, and manipulation. Both CMOS and CCD chips sense light through similar mechanisms, by taking advantage of the photoelectric effect, which occurs when photons interact with crystallized silicon to promote electrons from the valence band into the conduction band. Thus, the quality of the image that is captured is reflective of how much light and the manner in which the light reaches the light sensor (i.e., the photodiode). That is, parameters such as angle of incidence, light beam manipulation, and light beam filtering are important to control in order to ensure the capture of a high quality image that accurately reflects the true and correct image being captured.
One such parameter that affects the quality of the image captured is the amount of infrared light that reaches the light sensor. It is well known that visible light (to the human eye) has a wavelength range of 400 to 700 nm. Just beyond the visible light range is the infrared range that is defined by light having a wavelength in the range of 700 to 2500 nm. A subset of the infrared range is the near-visible infrared (NIR) range which is more of a concern to the digital imaging industry. The NIR range is defined by light having a wavelength in the range of 700 to 1200 nm. Particularly, in CMOS arrays, too much NIR light causes the captured image to appear washed out. That is, the contrast between the colors is not as sharp as it appears in real life. As such, it is important to filter out NIR light from visible light when capturing an image with a CMOS device or any other device that utilizes a light sensor to convert the intensity of incident light into a voltage signal.
In the past, interference or absorption IR filters have been designed and used in image-capturing devices to filter out infrared light from visible light in a number of different applications. Typically, an interference IR filter reflects IR light before it reaches the light-capturing device and an absorption IR filter absorbs the IR light before it reaches the light-capturing device. IR filters are designed to pass visible light having wavelengths below 700 nanometers while blocking infrared light having higher wavelengths extending into the near-infrared region (700 to 1200+ nanometers). Such an IR filter is often utilized to protect infrared-sensitive CMOS arrays typically incorporated in digital-imaging devices from infrared wavelengths. Thus, when an infrared filter is used within the optical path (i.e., the filter arranged such that incident light must pass through the filter in order to reach the light sensor), the negative effects of infrared light are reduced when capturing an image.
For example, FIG. 1 is a cutaway view of a conventional CMOS array 100 that is typically used in a conventional image-capturing device. The CMOS array 100 includes an IR filter 135 in the optical path between incident light 190 and each pixel 101, 102, and 103. The conventional CMOS array 100 includes a plurality of pixels 101, 102, and 103 arranged in columns and rows. The columns and rows are not shown for clarity; however, portions of the adjacent pixels 102 and 103 in the same row are shown in FIG. 1 to the left and to the right of pixel 101.
Each pixel 101 includes a photodiode 105 embedded in a silicon substrate 104 and each photodiode 105 is associated with electronic circuitry (not shown for clarity) contained in adjacent metal layers 110. Together, the photodiode 105 and its associated electronic circuitry in the metal layers 110 form a collection well 107 whereby incident light 190 may be directed toward the photodiode 105. In order to concentrate incident photons (from the incident light 190) into the collection well 107, the collection well 107 is capped by a miniature, positive-meniscus lens known as a microlens 120, or lenticular.
One particular kind of pixel 101 is a standard three-transistor pixel which is well known in the art and will not be discussed in detail herein. When a broad wavelength band of visible light 190 is incident on a pixel 101, a variable number of electrons are released from the semiconductor 104 in proportion to the photon-flux density incident on the surface of a photodiode. In effect, the number of electrons produced is a function of the wavelength and the intensity of light striking the semiconductor 104. Electrons are collected in a potential well (not shown) until an integration period is complete (as determined by the associated circuitry), and then the collected electrons are converted into a voltage signal. The voltage signal can then passed through an analog-to-digital converter (not shown in FIG. 1), which forms a digital electronic representation of the image, pixel by pixel, captured by the CMOS array 100.
The columns and rows of pixels 101 that form the CMOS array 100 are collectively covered by a cover glass 130 or a cover plate to form a shellcase package. The cover glass 130 fits securely on portions of the metal layer 110 over the array of pixels 101 such that a cavity 121 is formed over the microlens 120 of each pixel 101. In conventional CMOS arrays 100, this cavity 121 is filled only with air or may be a vacuum. Typically, the shellcase package is manufactured as a unit and then any modifications, such as adding an absorption filter 135, are accomplished during a separate manufacturing phase. As can be seen in the conventional CMOS array 100 of FIG. 1, an IR filter 135 is disposed on top of the cover glass 130. The IR filter 135 is designed to filter out or absorb light having wavelengths in the infrared range (i.e., greater than about 700 nm). As such, any NIR light within the incident light 190 will be filtered out before reaching the pixel 101 below.
Conventional IR filters 135 are commonly manufactured from dyed glass and comprise the most widely used types of filters for the attenuation of infrared light in digital-image-capturing devices. The absorption of specific wavelengths, that is, the filter's spectral performance, is a function of the physical thickness of the conventional IR filter 135 and the amount of dye present in the glass of the filter. Conventional IR filters 135 are made primarily from colored filter glass, and represent the largest class and most widely used type of filters for applications that do not require a precise definition of transmitted wavelengths. These conventional filters are commonly available in the form of glass, plastic-coated glass, acetate. Among the materials used in glass filters are the rare earth transition elements, colloidal dyes (such as selenide), and other molecules having high extinction coefficients that produce reasonably sharp absorption transitions.
Conventional absorption filters, such as absorption filter 135, are expensive and bulky and add to the overall depth of the optical path and bulk to the shellcase package. A typical absorption filter 135 is 10 microns thick which adds additional depth to the top of the shellcase package that includes the CMOS array 100. Further, the inclusion of a typical absorption filter 135 in a digital-image-capturing device requires an additional manufacturing step of affixing the absorption filter to the top of the glass cover 130. Because this manufacturing step is typically not performed in a clean room during the fabrication of the CMOS array 100, particulates and/or dust often may become embedded between the cover glass 130 and the absorption filter 135. Such dust and particulates may greatly affect the performance of the CMOS array 100 in the image-capturing device. Thus, the manufacturing complexity and assembly process adds to the cost and time required to produce a CMOS array 100 suitable for digital image-capturing devices.
Thus, it would be most beneficial to have a shellcase package with an integrated absorption filter that does not require the additional manufacturing steps associated with or the inherent drawbacks of conventional absorption filters 135 as shown in FIG. 1.